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Two major subsurface positive elements within and marginal to the Ouachita foldbelt in Texas, south and east of the foredeep basins, have been defined by recent drilling and seismic data.
Uplifted foreland carbonate rocks of early Paleozoic age were drilled on the Devils River uplift in southwest Texas in Shell 1 Stewart. This structure is interpreted to be a differentially uplifted autochthonous basement block partly overridden by the interior zone of the Ouachita foldbelt.
The presence of a second major linear positive element has been established in east-central Texas by seismic records and by Shell 1 Barrett. A faulted anticlinal structure, herein named the "Waco uplift," is at least 25 mi behind the leading edge of overthrust Ouachita facies rocks. A massive section of intensely deformed foreland carbonate rocks of early and middle(?) Paleozoic age, overlain by allochthonous low-grade metaclastic rocks of Ouachita facies, was drilled in the Barrett well. The uplift appears to be similar in structural position and history to the Devils River uplift, but the Waco uplift is probably a parautochthonous block which was partly involved in late Paleozoic orogenic thrusting.
The Waco and Devils River uplifts are subregional elements superimposed on a broad elongate gravity maximum which can be traced from the Ouachita Mountains to near the Marathon area. This maximum, thought to be the result of uplift of the dense mafic crustal layer, may have marked the edge of the early and middle Paleozoic (preorogenic) continent. A well-defined, persistent, depositional and tectonic boundary between the constructional carbonate miogeocline and thinner contemporaneous pelitic deposits of the geosyncline was also apparently localized along or near the periphery of this inferred crustal margin. Therefore, the two uplifts are interpreted to be external massifs in structural and stratigraphic positions similar to those of the Green Mountains-Blue Ridge chain of basement u lifts in the Appalachians.
Isotopic dates from both Ouachita facies and foreland rocks of the cratonic margin record multiple Paleozoic thermal events and probable accompanying deformation along the tectonic belt. The data also confirm the ancestral middle, and possibly early, Paleozoic origin of the Devils River and Waco uplifts. The presence of a post-Precambrian, pre-Late Cambrian volcanic and metasedimentary province in the area of the Devils River uplift is suggested by Rb-Sr dating of rocks drilled in the 1 Stewart well.
Foreland strata in the 1 Stewart are essentially undeformed, but the rocks in the 1 Barrett are metamorphosed to greenschist facies and highly contorted and sheared. Some formation fluids and minor hydrocarbon shows were recovered from these tests, but the quality of the reservoirs was generally poor.
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